The present invention relates to a method of manufacturing structural elements in laminated material, so-called sandwich elements, in which a core material is provided with a covering layer on at least one side. 2. History of the Related Art
It is customary to produce such elements, by adhering to the core, which is of cellular plastics or other suitable material and advanced horizontally, the covering layer, which is of plastics, sheet metal or other suitable material. In such sandwich structures, produced by adhering cores and covering layers, there are problems due to the air or other gas being trapped in the surface cells of the upper and lower sides of the core material. Seen in magnification, these cells appear as approximately hemispherical depressions. When the upper surface of the core is wetted by adhesive, surface tension in the latter causes a lid to form over the surface cell, and large amounts of air or other gas are trapped under the lids. Since the core materials normally used are proof against diffusion the trapped gas can not depart from the underside of the core either. On the other hand, if a core with diffusion properties is used, this only moves the source of the problem from the upper to the underside of the core without eliminating the described entrapment of gas. During the curing or hardening process the encapsulated gas may remain in the surface cell and reduce adhesive contact with the cellular plastics, mix with the adhesive to give the latter a porosity considerably reducing its strength, or diffuse through it to lie as a gas film between adhesive and covering layer, thus eliminating contact between the two.
The described problem results in that the adhesive and the adhesive grip on the core or covering material can be reduced by up to 90%. For heavy static or dynamic loading or heat stresses, e.g. from solar radiation or cleaning at an increased temperature, in such a structural element there will be delamination somewhere in the adhesive layer between covering layer and core due to the air or other gas encapsulations, even though the covering layer, adhesive and core material are such sufficiently strong, suitable and well-prepared for the structural adhesion in question.
In curing or hardening processes that generate heat the encapsulated air or gas will also be heated and thus caused to expand. There is then a very great risk that the lamination is disturbed in a very sensitive stage in its fixed position against the core, and the adhesive joint will thus be partially or totally inhibited over large surfaces.
In the known horizontal processes, a solution to these problems has been attempted by laminating one side of the core at a time, thus requiring it to be turned over between lamination steps. Another method would be to remove the gas by chemical methods. However, these solutions afford unsatisfactory results, and they also make the manufacturing process more expensive and consequently the finished product. Nor is it possible with the known processes to provide a continuous manufacturing process that gives an adhesive joint between diffusion-proof cores and covering layers which is free from air or gas.